1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
In general, an electrographic image forming apparatus irradiates (exposes) charged photosensitive drums with laser light based on image data so as to form electrostatic latent images thereon, and makes toners made of colored powders adhere to the electrostatic latent images so as to visualize the electrostatic latent images and accordingly form toner images on sheets of paper.
In an electrographic image forming apparatus, glossiness of images formed on sheets changes as a gradation level changes. This is because, in the electrographic system, gradations of color toner images, such as Y (yellow) toner images, M (magenta) toner images, C (cyan) toner images and K (black) toner images, are expressed by area coverage modulation (screens or halftone dots), and accordingly roughness exists on the surfaces of images (image surface) microscopically, and the roughness on the image surfaces changes as the gradation level changes.
In recent years, there has been proposed an image forming apparatus which uses a clear toner in addition to color toners so as to increase glossiness of images.
However, on a sheet on which color toner images (or a color toner image) are formed, as described above, roughness exists on the image surface because of the amounts of toners adhering to the sheet (toner adhesion amount) different depending on input image data (density gradation data), the amounts of toners adhering to the sheet different depending on colors (Y, M, C and K) for the same gradation, or the like. Hence, if a clear toner is made to adhere onto such a sheet uniformly, gloss unevenness is generated. The gloss is detected from specular reflection light reflected from the image surface on the sheet. Therefore, in order to remove or reduce the gloss unevenness, it is important to increase smoothness of the image surface on a sheet.
Then, for example, there is disclosed in Japanese Patent Application Laid-Open Publication No. hei 9-200551 (Patent Document 1) a digital color copying apparatus which forms a clear toner image on the basis of an inversion signal of an input image signal so as to correct the gloss unevenness.
Further, there is disclosed in Japanese Patent Application Laid-Open Publication No. 2005-275250 (Patent Document 2) an image forming apparatus provided with a glossiness sensor to detect glossiness on a recording sheet after fixing, the image forming apparatus which obtains a clear toner development-glossiness characteristic, and determines a development contrast to attain target glossiness.
Further, there is disclosed in Japanese Patent Application Laid-Open Publication No. 2007-183593 (Patent Document 3) an image forming apparatus provided with a glossiness sensor to detect glossiness on a recording sheet after fixing, the image forming apparatus which detects glossiness of each gradation using a clear toner and color (Y, M, C and K) toners, and controls the amount of the clear toner per unit area on the recording sheet in such a way that, with respect to each of Y, M, C and K, the glossiness is uniform at the highest glossiness among glossiness of gradations of 0 to 255. In the image forming apparatus disclosed in Patent Document 3, the control of the amount of the clear toner is performed by changing a gradation level.
However, the toner adhesion amount of a color image depends on a gradation-toner adhesion amount characteristic of each of Y, M, C and K. Hence, the total toner adhesion amount of the color toners and the clear toner on a sheet corrected by the clear toner on the basis of the inversion signal of the input image signal is not uniform, so that the technology disclosed in Patent Document 1 cannot solve the gloss difference (gloss unevenness). In addition, even when the total toner adhesion amount corrected by the clear toner is uniform, if the total height of the color toner images and the clear toner image is not uniform, roughness is produced on the image surface, and accordingly the gloss difference is generated. For example, even if the toner adhesion amounts of Y, M, C and K are the same, the heights of Y, M, C and K toner images (toner image height) are different from each other because of the different specific gravity of Y, M, C and K toners.
Further, although the technology disclosed in Patent Document 2 can reduce the gloss difference between an image portion and a non-image portion, the technology cannot reduce the gloss unevenness in the image portion because the clear toner is developed uniformly on an image.
Further, although the technology disclosed in Patent Document 3 can make the glossiness uniform in an image portion of each of Y, M, C and K regardless of gradations, the technology generates the gloss/density unevenness among Y, M, C and K. In addition, as the number of colors (Y, M, C and K) laid on top of each other increases, namely, from a primary color to a secondary color, a tertiary color and then a quartic color, the gloss difference increases, and accordingly the roughness on the surface of the recording sheet becomes noticeable. The technology disclosed in Patent Document 3 has the following problems (1) to (3) too.
(1) In a case where the amount of the clear toner is changed only by changing the gradation level without area coverage modulation (without screening), the number of levels to change the toner adhesion amount by processing decreases, and also the clear toner image is uniformly formed all over the screens (halftone dots) of the color toner images. Accordingly, the image surface of the color toner images on which the clear toner image is formed (the image surface of the color toner images plus the clear toner image) is not always smooth. In order to make the image surface smooth, it is necessary to increase the amount of the clear toner. Accordingly, the consumption of the clear toner increases.(2) In a case where the amount of the clear toner is changed with area coverage modulation (with screening), the number of levels to change the toner adhesion amount increases, but the halftone dots of the clear toner image are laid on the halftone dots of the color toner images. Accordingly, the image surface is not smooth. In addition, when color shift (shifts in printing positions of toners) occurs, the roughness on the image surface of the color toner images plus the clear toner image changes. Accordingly, images cannot be stably outputted.(3) In the case where the amount of the clear toner is changed with area coverage modulation (with screening), which is described in (2), an improvement can be made with respect to the printing positions of the color toner images and the clear toner image by normal resist control. However, the resist control is performed on photosensitive drums or a transfer belt. Accordingly, when images are formed on sheets, the printing positions may be shifted thereon by a load or the like for sheet conveyance onto the transfer unit or the like.
As a result, the technology disclosed in Patent Document 3 has a problem that if a clear toner image is formed on color toner images, a large amount of a clear toner is consumed, or desired gloss cannot be made because of the color shift.